There has been a problem that, when a diamond coat (or film) is applied to a carbide base material by gas phase synthesis, (termed hereinafter as "vapor deposition technique"), the diamond coat is easy to peel off due to differences in hardness, thermal expansion coefficient and crystal structure between the carbide base material and diamond coat and further to insufficient strength in adhesion of the diamond coat to the base material. Therefore, various proposals have been made to improve the adhesion strength of diamond coats. However, a satisfactory adhesion strength of the diamond coat to the base material for use in cutting tools to which great mechanical impact or thermal stress is applied has not been achieved so far, and there are cases where the diamond coating is not applicable to a tool such a drill as having a complicated shape.
Main proposals to improve the adhesion strength of the diamond coat include methods to obtain a mechanical engagement by having bumps and dips formed on the surface of a base material, to have an intermediate layer developed having a small coefficient of thermal expansion relative to that of diamond, to provide machining distortion with the aim of giving residual stress to the surface of the base material or to perform shot peening or ion implantation.
As a means to have bumps and dips formed on the surface of base materials, for example, a method of chemical etching using acids and alkali has been disclosed in JP Patent Kokai JP-A-62-67174/1987 or JP-A-5-179450/1993, a method of growing crystal grains or providing bumps and dips by allowing a surface reformed layer by heat treatment to develop in JP-A-01-246361/1989 or JP-A-6-191993/1994, a method of grinding machining in JP-A-63-14869/1988, a method of machining base materials by a laser in JP-A-5-311442/1993, a method of etching using ion or plasma in JP-A-60-204695/1985 or JP-1-145396/1989, a method of electrolytic etching in JP-A-3-146663/1991, JP-A-3-107460/1991, JP-A-3-183774/1991, JP-A-4-221075/1992 and JP-A-2-217398-1990, a method of soaking base materials in a solvent in which abrasive grains of diamond are dispersed and simultaneously performing ultrasonic washing and flawing treatment in JP-A-62-226889/1987, respectively.
Also, a method of providing machining strain has been disclosed in JP-A-61-270372/1986, however, in this method, an adhesion effect is reduced due to relation of residual stress caused by an increase in a temperature at the time of diamond synthesis and, in addition, the adhesive strength of diamond coat fluctuates widely because of poor reproducibility of the machining strain.
As a means to have an intermediate layer developed, a method of having an intermediate layer formed of carbide or nitride of IV, V, VI group metals grown on the surface of base material has been disclosed in JP Patent Kokai JP-A-58-126972/1983, however, in this method, the improvement of adhesive strength has not been achieved up to a practicable level because chemical bonding between diamond and the intermediate layer is hard to occur.